2) Shift+W in Object Mode (add Mirror modifier) doesn’t work as expected. It removes one half of object after the first use. And removes a second half after the second use. And then do nothing. It doesn’t add modifier at all. And I can’t figure out why.

Hi. Thanks! Glad you’re enjoying it. [GENERIC GREETINGS MODE DISABLED/SERIOUS RESPONSE MODE ENGAGE] Do not use Sensei Format with test builds of Blender (or do at your own risk). This is mentioned in nearly every video and document on installing Sensei Format.

As far as the alt right click disabling everything. I’m not sure if that’s possible. Doesn’t seem to be the case with anyone else’s install, so I’m not sure how to address this issue, could be related to the test build you’re using. Try with an official (current stable release from the official download page blender.org/download) install of Blender, then let me know if it’s still happening.

The Sensei Format hotkey “Shift W” for mirror modifiers has been updated. See page 3 of the Sensei Format hotkeys manual to learn more: https://blendersensei.com/sensei-format-hotkeys/ I went ahead and made a note about this in the Crash Course and on the Youtube video which should help make people aware of it until a new video is posted. Thanks for reporting these issues.